Security on New York City subways has been tightened again following another series of explosions in London, two weeks after the deadly attacks earlier this month.

British police say they're questioning two men who were arrested a few hours after Thursdays bombings.

One man was arrested near Downing Street, the site of Prime Minister Tony Blair's residence, and the other was detailed near Tottenham Court Road, which is near the Warren Street subway station where one of the incidents took place.

Four small blasts forced the evacuation of three Tube stations and a double-decker bus early Thursday afternoon, London time, but only one person was reported injured.

In New York City, officials announced Thursday afternoon that the NYPD is instituting random searches of subway passengers carrying bags or backpacks, and similar searches may be conducted aboard city buses.

"It might slow individuals down, but we will do it in a reasonable way," said Police Commissioner Raymond Kelly at a press conference with Mayor Michael Bloomberg shortly after noon.

The random searches will be instituted across the entire subway system starting Friday. Spot searches have reportedly begun already in selected areas.

Kelly said that ideally, straphangers will be searched before they pass through the turnstiles, although they could also be searched once inside the system.

Bloomberg stressed that the heightened security is not the result of a known threat. "There are still no threats to this city that have been explicitly made to our subway or bus system," the mayor said. "People should go ahead and feel comfortable using it."

Kelly said officers on Wednesday's overnight shift were held over till noon Thursday, in order to boost the uniformed presence on the subways following the latest London attack.

Terrorism experts say the explosives used Thursday appear less sophisticated than the ones used in the deadly attacks that hit the British capital two weeks ago.

That could suggest they were a copycat operation, but experts say it's too early to tell.

London police commissioner Ian Blair says some of the explosive devices apparently didn't go off. He says there's evidence at the scene that could give police a significant break in the case.

The commissioner says it is not clear whether Thursday's attacks at the Warren Street, Shepherd's Bush and Oval stations are connected to the July 7 bombings. However, he said they have a "similar pattern to the previous ones."

British Prime Minister Tony Blair appeared shortly after Thursday's explosions and urged the public to remain calm. Afterward he held an emergency Cabinet meeting but said no decisions "of a policy nature" were made.

"We can't minimize incidents such as this," Blair said at a joint news conference with the Australian prime minister at No. 10 Downing St. "They're done to scare people, to frighten them and make them worried."

Senior aides to President Bush briefed him on the attacks shortly before he gave an address to members of the Organization of American States. The president said that terrorists are trying to shake the will of the free world by killing innocent people.

"They don't understand that when it comes to the defense of universal freedoms, this country won't be frightened," Bush said.